Microfluidic elements for thoroughly mixing a liquid with a reagent are used, for example, in diagnostic tests (in vitro diagnostics), using the microfluidic elements bodily fluid samples are analyzed for an analyte contained therein for medical purposes. The term thoroughly mixing comprises both the possibility that the reagent is provided in liquid form, i.e., that two liquids are mixed with one another, and also that the reagent is provided as a solid and is dissolved in a liquid and homogenized. An important component of the analysis is a so-called test carrier, on which, for example, microfluidic elements having channel structures for receiving a liquid sample (in particular a bodily fluid) are provided, to allow the performance of complex multistep test protocols. A test carrier can comprise one or more microfluidic elements.
For example, in immunochemical analyses having a multistep test sequence, in which a separation of bound and free reaction components occurs (“bound/free separation”), fluidic test carriers are used, using which a controlled liquid transport is possible. The control of the fluidic process sequence can be performed using internal measures (inside the fluidic element) or using external measures (e.g., provided in the device). The (external) control can be based on the application of pressure differentials or a change of forces, the latter being able to result from the change of the action direction of gravity, for example, but also from centrifugal forces which act on a rotating microfluidic element or a rotating test carrier and are a function of the rotational velocity and the distance from the axis of rotation, for example.
Microfluidic elements and also test carriers of this type comprise a carrier material, typically made of a substrate made of plastic material. The elements and test carriers have a sample analysis channel enclosed by the substrate and a cover or a cover layer, which often comprises a sequence of multiple channel sections and chambers lying between them, which are widened in comparison to the channel sections. The structures and dimensions of the sample analysis channel having its channel sections and chambers are defined by structuring of plastic parts of the substrate, which are generated, for example, by injection molding techniques or other methods for producing suitable structures.
To perform the analyses, the sample analysis channel contains a reagent which reacts with a liquid introduced into the sample analysis channel. The liquid sample and the reagent are mixed in the test carrier with one another in such a manner that a reaction of the sample liquid with the reagent results in a change of a measured variable which is characteristic for the analyte contained in the sample liquid. The measured variable is measured on the test carrier itself. Above all, optically analyzable measuring methods are typical, in which a color change or another optically measurable variable is detected.
Predominantly laminar flow conditions prevail in the sample analysis channel having its capillary channel structures and small dimensions. Liquids and/or liquids and solids mix thoroughly only poorly in such capillary channels. Multiple procedures are known in the prior art for improving the thorough mixing of reagent and sample liquid.
For example, in rotating test carriers which are rotated around a rotation axis in an analysis system, the thorough mixing is encouraged by rapid changes of the rotational direction or by changing the rotational velocity. This “shake mode” places high demands on the drive unit of the analysis system, however. The wear and the danger of occurring malfunctions and breakdowns are comparatively greater.
A further method known in the prior art for improving the thorough mixing of sample liquid and reagent is the introduction of magnetic particles which are set into motion by the action of an electromagnetic or permanent magnet. The outlay in the production of the test carrier rises due to the integration of the particles. In addition, the analysis systems must have a further component, namely the magnets.
Furthermore, elements are known whose capillary channels contain special flow obstructions, such as ribs. The production of obstructions of this type, which are often implemented as a microstructure, makes the production process of the test carrier more costly and difficult. In addition, structures of this type are not suitable for all mixing processes and/or for all reagents and sample liquids.
In spite of the many attempts to improve mixing procedures and microfluidic elements, such as test carriers, in particular the thorough mixing of reagent and sample liquid, there is a further need for a microfluidic element improved in this regard.